The low performance of Chile in the TIMSS 1998/99 international study of mathematics and science achievement was a great disappointment for that country. To investigate the likely causes for low performance in mathematics, this study (1) compared Chile to three countries and one large school system that had similar economic conditions but superior mathematics performance, and (2) examined how important characteristics of the Chilean educational system could account for poor student achievement in mathematics. The study finds that, compared to South Korea, Malaysia, the Slovak Republic, and Miami-Dade County Public Schools: (a) Chilean 8th graders had parents with fewer years of schooling and with fewer educational resources at home; (b) the Chilean mathematics curriculum covered less content and fewer cognitive skills; and (c) the meager official curriculum translated into a weaker curricular implementation. Hierarchical linear models found that, in Chile, school assets were unequally distributed across social classes, with schools in socially advantaged areas more likely to have their own mathematics curriculum and better prepared teachers who emphasized more advanced mathematics content. Schools with their own mathematics curriculum and whose teachers covered more advanced content had significantly higher student achievement in mathematics.